In United Kingdom patent specification No. 1 385 670, there is described a gas administration apparatus on which one or more gas flow units can be mounted in a removable plug-in fashion. This known plug-in system has proved to be very popular in that it simplifies the installation and removal of gas flow units from the apparatus thereby facilitating maintenance and cleaning of the units as well as the replacement of a unit should it fail during an operation.
It is common practice for an anaesthesia apparatus to have mounted on it two or more separate vaporisers for delivering different volatile anaesthetics so that the same basic anaesthesia apparatus can be used during a series of surgical operations to meet the needs of different patients. In UK patent No 2 052 271, there is described an interlocking system which prevents two different anaesthetics being delivered to a patient at the same time or otherwise becoming mixed. This known interlocking system includes pins associated with each vaporiser which, when its concentration dial is moved from the off position, causes the pins to extend outwardly and block the movement of corresponding pins on immediately adjacent vaporisers thereby preventing the concentration dials of these other vaporisers from being operated. With this arrangement it is impossible to switch more than one vaporiser into a gas circuit at any one time.
This known interlocking system works well when there are two or more units arranged side-by-side. However, if the middle unit of, for example, three units is removed for any reason then the interlocking action of the first unit cannot be transmitted to the third unit and vice versa. This problem has been overcome by using non-functional (dummy) second units or merely applying warning labels to indicate that no interlocking system is available.